Message transmission in wireless access system

ABSTRACT

A method for transmitting messages to alter the operating status between a mobile station and a base station in a wireless access system comprises, at the mobile station, transmitting a first message to the base station, setting a first count to a first predetermined value and enabling a first, the first message requesting a alteration in operating status between the mobile station and the base station, whereupon expiration of the first timer before receiving a second message from the base station, the count is decremented and, if the count is above a second predetermined value, the first timer is reset and the first message is retransmitted to the base station. The method also comprises, at a base station, receiving the first message, transmitting a second message to the mobile station and enabling a second timer, whereupon expiration of the second timer, the alteration in operating status is effected.

CROSS-REFERENCE TO RELATED APPLICATIONS

Pursuant to 35 U.S.C. § 119(a), this application claims the benefit ofearlier filing date and right of priority to Korean Application No.2004-078858, filed on Oct. 4, 2004 and No. 2005-0004248, filed on Jan.17, 2005, the contents of which are hereby incorporated by referenceherein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to a wireless access system and,more particularly, to message transmission in a wireless access system.

BACKGROUND OF THE INVENTION

Generally, the system of IEEE802.16e in progress of internationalstandardization for the broadband wireless access system consists of amobile station (MS), a base station (BS) and an ASA (authenticationservice authorization) as an authentication management server. In thiscase, a common physical layer (PHY) and medium access control (MAC)layer are defined between the MS and the BS.

1. Scan

FIG. 1 is diagram of an exemplary format of a scan request (e.g.,MOB-SCN-REQ) message. FIG. 2 is diagram of an exemplary format of a scanresponse (e.g., MOB-SCN-RSP) message. An MS requests a scanning intervalfrom a currently accessed serving BS to measure a signal quality of aneighboring BS and to decide a target BS that will be accessed inhandover. The request is performed via a scan request (MOB-SCN-REQ)message illustrated in FIG. 1. In response to the request, the servingBS transmits a scan response (MOB-SCN-RSP) message, as illustrated inFIG. 2, including information associated with scan to the MS.

2. Sleep Mode

A broadband wireless access system supports a sleep mode to minimizepower consumption of an MS. An MS in sleep mode acts according to asleep interval that is increased by a fixed rate. The sleep intervalconsists of a listening window and a sleep window.

A value of the listening window is fixed via a sleep request/responsemessage. For the listening window, the MS is instructed via a broadcastMAC management message (MOB-TRF-IND) transmitted from the BS to indicatewhether there exists downlink traffic intended for the MS and whetherranging needs to be performed for uplink communication maintenance andappropriate downlink coding type maintenance according to signalquality. The sleep window is an interval during which the MS receivesminimal downlink signals from the BS in order to save power. For thesleep window, the MS performs a scanning of neighboring BSs forhandover, a ranging for uplink communication maintenance and appropriatedownlink coding type maintenance according to signal quality.

FIG. 3 is diagram of an exemplary format of a sleep request (e.g.,MOB-SLP-REQ) message. FIG. 4 is diagram of an exemplary format of asleep response (e.g., MOB-SLP-RSP) message. Initialization of the sleepmode is performed by exchanging a sleep request (MOB-SLP_REQ) messageand a sleep response (MOB-SLP-RSP) message between the MS and the BS.Specifically, to enter the sleep mode from a normal operation mode, theMS transmits a sleep request (MOB-SLP-REQ) message, as illustrated inFIG. 3, to the BS. The BS then transmits a sleep response (MOB-SLP-RSP)message, as illustrated in FIG. 4, to the MS.

3. Idle Mode

Idle mode enables an MS to save power consumption by receiving only aperiodically transmitted downlink broadcast traffic message(MOB-PAG-ADV) when moving through an area controlled by several BSswithout registering with a specific BS. In the idle mode, the MS has nosignaling requirement for handover. By restricting an action associatedwith scanning, the MS may save resources, such as power and a connectionID.

FIG. 5 is diagram of an exemplary format of a de-registration request(e.g., DREG-REQ) message. Referring to FIG. 5, the MS transmits ade-registration request (DREG-REQ) message to the BS for idle modeinitialization. In response to the message, the BS then transmits ade-registration command (DREG-CMD) message including informationassociated with idle mode admission of the MS. After transmitting theDREG-CMD message to the MS, the BS releases management resources such asa basic connection for MAC management message exchange with the MS, aprimary connection, and a secondary connection for standard-basedmessages (DHCP, TFTP, etc.).

FIG. 6 is diagram of an exemplary format of a de-registration command(e.g., DREG-CMD) message. As such, FIG. 6 illustrates action codesincluded in the DREG-CMD message. An MS performs an action according toan instruction indicated by the action code. For instance, if a code is0X05, an MS performs a de-registration procedure in a corresponding BSand initializes an idle mode. Moreover, even if the MS does not requestthe DREG-REQ message, the BS may transmit a DREG-CMD (unsolicitedDREG-CMD) message having an action code 0X05 to cause the MS to enterthe idle mode.

As mentioned in the above explanation, the MS transmits one of a scanrequest (MOB-SCN-REQ), a sleep request (MOB-SLP-REQ) and ade-registration request (DREG-REQ) message to the BS and then awaits oneof a scan response (MOB-SCN-RSP), a sleep response (MOB-SLP-RSP) and ade-registration command (DREG-CMD) message from the BS in response.

FIG. 7 is a signal flow diagram illustrating an exemplary case wherescan request and response messages are lost. FIG. 8 is a signal flowdiagram illustrating an exemplary case where sleep request and responsemessages are lost. FIG. 9 is a signal flow diagram illustrating anexemplary case where de-registration request and command messages arelost. Referring to FIGS. 7-9, due to the limited radio link resourcesand wireless environmental factors, the MOB-SCN-REQ, MOB-SLP-REQ andDREG-REQ messages transmitted from the MS may not be delivered normallyto the BS or the MOB-SCN-RSP, MOB-SLP-RSP and DREG-CMD messagestransmitted from the BS in response may not be delivered normally to theMS.

In such case, since a processing procedure for a next action of the MSis not defined in a current broadband wireless access system, it ishighly probable that the MS or BS may perform a wrong action.

In the idle mode, the BS releases the connections (basic, primary andsecondary connections) and transmits the DREG-CMD message to the MS.Therefore, if the MS fails to receive the DREG-CMD message andretransmits the DREG-REQ message, there is no available connection(basic connection). Furthermore, even if the MS does not request idlemode initialization via the DREG-REQ message, if the DREG-CMD(unsolicited DREG-CMD) message transmitted to cause the MS to enter theidle mode is lost, the MS continues to operate in a normal operationmode. However, after deciding that the MS has entered the idle mode, theBS releases management resources for the corresponding MS. Thereafter,the MS is unable to transmit messages to the BS.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to message transmissionin a wireless access system that substantially obviates one or moreproblems due to limitations and disadvantages of the related art.

An object of the present invention is to provide for transmission ofmessages between a mobile station and a base station in a wirelessaccess system.

Another object of the present invention is to provide for transmissionof a valid message in a case where a message is lost due to limitedradio link resources and/or environmental radio factors.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, inone embodiment, a method for transmitting messages to alter theoperating status between a mobile station and a base station in awireless access system comprises, at the mobile station, transmitting afirst message to the base station, setting a first count to a firstpredetermined value and enabling a first, the first message requesting aalteration in operating status between the mobile station and the basestation, whereupon expiration of the first timer before receiving asecond message from the base station, the count is decremented and, ifthe count is above a second predetermined value, the first timer isreset and the first message is retransmitted to the base station. Themethod also comprises, at a base station, receiving the first message,transmitting a second message to the mobile station and enabling asecond timer, the second message acknowledging the mobile stationrequest for an alteration in operating status, whereupon expiration ofthe second timer, the alteration in operating status is effected andupon receiving another first message, the second message isretransmitted to the mobile station and the second timer is reset.

The first message may be a de-registration request (DREG-REQ), the firsttimer is a de-registration command standby timer, the first count is ade-registration request retransmission count, the alteration inoperating status is a transition to an idle mode, the second message isa de-registration command (DREG-CMD), and the second timer is amanagement resource holding timer. The method may further comprise, atthe mobile station, performing a scan operation in response to receivingthe second message from the base station. The method may furthercomprise, at the mobile station, entering idle mode in response toreceiving the second message from the base station.

In another embodiment, a method for altering the operating statusbetween a mobile station and a base station in a wireless access systemcomprises receiving a first message, the first message requesting thealteration in operating status. The method also comprises transmitting asecond message, the second message acknowledging the request for analteration in operating status. The method also comprises enabling atimer, and upon expiration of the timer, effecting the alteration inoperating status and upon receiving another first message,retransmitting the second message and resetting the timer.

The first message may be a de-registration request, the alteration inoperating status is a transition to an idle mode, the second message isa de-registration command, and the timer is a management resourceholding timer.

In yet another embodiment, a method for altering the operating statusbetween a mobile station and a base station in a wireless access systemcomprises transmitting a first message, the first message indicating thealteration in operating status. The method also comprises enabling afirst timer and setting a count to a first predetermined value. Themethod also comprises enabling a second timer, whereupon expiration ofthe first timer before receiving a second message, the count isdecremented and, if the count is above a predetermined value, the firsttimer is reset and the first message is retransmitted and, whereuponexpiration of the second timer, the alteration in operating status iseffected.

The first message may be an unsolicited de-registration command, thealteration in operating status is a transition to an idle mode, thesecond message is one of a de-registration request and a de-registrationacknowledgment, the first timer is a de-registration response standbytimer, and the second timer is a management resource holding timer.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings. It is to be understood that both the foregoinggeneral description and the following detailed description of thepresent invention are exemplary and explanatory and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is diagram of an exemplary format of a scan request (e.g.,MOB-SCN-REQ) message.

FIG. 2 is diagram of an exemplary format of a scan response (e.g.,MOB-SCN-RSP) message.

FIG. 3 is diagram of an exemplary format of a sleep request (e.g.,MOB-SLP-REQ) message.

FIG. 4 is diagram of an exemplary format of a sleep response (e.g.,MOB-SLP-RSP) message.

FIG. 5 is diagram of an exemplary format of a de-registration request(e.g., DREG-REQ) message.

FIG. 6 is diagram of an exemplary format of a de-registration command(e.g., DREG-CMD) message.

FIG. 7 is a signal flow diagram illustrating an exemplary case wherescan request and response messages are lost.

FIG. 8 is a signal flow diagram illustrating an exemplary case wheresleep request and response messages are lost.

FIG. 9 is a signal flow diagram illustrating an exemplary case wherede-registration request and command messages are lost.

FIG. 10 is a signal flow diagram illustrating scan request and responsemessages using a scan response standby timer, according to an embodimentof the present invention.

FIG. 11 is a signal flow diagram illustrating sleep request and responsemessages using a sleep response standby timer, according to anembodiment of the present invention.

FIG. 12 is a signal flow diagram illustrating de-registration requestand command messages using a de-registration command standby timeraccording to an embodiment of the present invention (e.g., a case wherethe de-registration request message transmitted from a mobile station islost).

FIG. 13 is a signal flow diagram illustrating de-registration requestand command messages using a de-registration command standby timeraccording to another embodiment of the present invention (e.g., a casewhere the de-registration command message transmitted from a basestation is lost).

FIG. 14 and FIG. 15 are flow diagrams illustrating a method according toanother embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

The present invention is implemented in a wireless access system, suchas a broadband wireless access system. The present invention is alsoapplicable to wireless communication systems operating according toother standards.

The present invention proposes a method in a wireless access system foroptimizing scan request (MOB-SCN-REQ) and scan response (MOB-SCN-RSP)messages, such that a mobile station (MS) may measure a channel qualityof a neighboring base station (BS) for handover, sleep request(MOB-SLP-REQ) and sleep response (MOB-SLP-RSP) messages exchangedbetween an MS and a BS to initialize a sleep mode, and de-registrationrequest (DREG-REQ) and de-registration command (DREG-CMD) messagesexchanged between an MS and a BS to initialize an idle mode.Specifically, the present invention may provide a valid MS-to-BS messagetransmitting procedure if a message is lost due to limited radio linkresources and/or environmental radio factors.

In the present invention, the MS performs retransmission of the requestmessages by operating response standby timers for scan, sleep andde-registration request messages and setting the request retransmissioncount of the messages.

First of all, the present invention carries out a retransmission of thescan request (MOB-SCN-REQ) message by setting a scan response standbytimer and a scan request retransmission count when performing a scanoperation.

The MS sets a scan response standby timer upon transmitting theMOB-SCN-REQ message to the BS. If the MOB-SCN-RSP message is notreceived from the BS prior to expiration of the timer, the MS considersthe MOB-SCN-REQ or MOB-SCN-RSP message lost, e.g., lost over a radiolink, and retransmits the MOB-SCN-REQ message to the BS if the scanrequest retransmission count is not exhausted.

Table 1 illustrates an exemplary drive time of a scan response standbytimer and a scan request retransmission count according to the presentinvention.

TABLE 1 System Name Action Value MS Scan response Time for standby until1 second standby timer retransmitting scan request message if scanresponse message not received Scan request Available retransmission 3times retransmission count of scan request message count

Furthermore, an embodiment of the present invention may perform aretransmission of the sleep request (MOB-SLP-REQ) message by setting asleep response standby timer and a sleep request retransmission countwhen initializing sleep mode.

The MS sets the sleep response standby timer upon transmitting theMOB-SLP-REQ message to the BS. If the MOB-SLP-RSP message is notreceived from the BS within a specific timer time, the MS considers theMOB-SLP-REQ or MOB-SLP-RSP message lost, e.g., lost over radio link, andretransmits the MOB-SLP-REQ message to the BS if the sleep requestretransmission count is not exhausted.

Table 2 illustrates an exemplary drive time of a sleep response standbytimer and a sleep request retransmission count according to the presentinvention.

TABLE 2 System Name Action Value MS Sleep response Time for standbyuntil 1 second standby timer retransmitting scan request message ifsleep response message not received Sleep request Availableretransmission 3 times retransmission count of sleep request messagecount

Moreover, the present invention performs a retransmission of thede-registration request (DREG-REQ) message by setting a de-registrationcommand standby timer and a de-registration request retransmission countupon initializing idle mode.

The MS sets the de-registration command standby timer upon transmittingthe DREG-REQ message to the BS. If the DREG-CMD message is not receivedfrom the BS within a specific timer time, the MS considers the DREG-REQor DREG-CMD message lost, e.g., lost over radio link, and retransmitsthe DREG-REQ message to the BS if the de-registration requestretransmission count is not exhausted.

Table 3 illustrates an exemplary drive time of a de-registration commandstandby timer and a de-registration request retransmission countaccording to the present invention.

TABLE 3 System Name Action Value MS De-registration Time for standbyretransmitting de- 1 second command registration request message if de-standby timer registration message not received De-registrationAvailable retransmission count of 3 times request de-registrationrequest message retransmission count

The BS sets a management resource holding timer upon transmitting theDREG-CMD message to the MS and maintains connections for a predeterminedtime. If the management resource holding timer expires, the BS releasesthe held management resources.

Table 4 illustrates an exemplary management resource holding timeraccording to the present invention.

TABLE 4 System Name Action Value MS Management Time for holdingmanagement 1 resource resources after transmitting de- second holdingregistration command message to timer MS before releasing heldmanagement resources

Table 5 illustrates an exemplary drive time of a de-registrationresponse standby timer and a de-registration command retransmissioncount according to the present invention.

TABLE 5 System Name Action Value BS De-registration Time for standbyuntil receiving 1 second response response message after transmittingstandby de-registration command message timer De-registration Availableretransmission count of 3 times command de-registration command messageretransmission count

Table 6 illustrates a de-registration request code for a response with aDREG-REQ message according to the present invention if an MS receives anunsolicited DREG-CMD message not requested from a BS.

TABLE 6 Size Syntax (bits) Notes DREG-REQ message format( ){ Management8 message type=49 De-registration 8 0x00: MS de-registration requestfrom Request Code BS and network 0x01: request for MS de-registrationfrom Serving BS and initiation of MS Idle mode 0x02: Acknowledgement forunsolicited DREG-CMD 0x03-0xFF: Reserved TLV encoded Variablesparameters }

Table 7 illustrates a message format for a response with a DREG-ACKmessage according to the present invention if an MS receives anunsolicited DREG-CMD message not requested from a BS.

TABLE 7 Syntax Size (bits) Notes DREG-ACK message format( ){ Managementmessage 8 type=xx Action Code 8 This indicates Action code from the BSHMAC tuple Variables }

FIG. 10 is a signal flow diagram illustrating scan request and responsemessages using a scan response standby timer, according to an embodimentof the present invention.

Referring to FIG. 10, an MS enables a scan response standby timer upontransmitting a scan request (MOB-SCN-REQ) message to a BS. If a scanresponse (MOB-SCN-RSP) message is not received before the scan responsestandby timer expires, the MS checks whether the scan requestretransmission count is exhausted. If the scan request count is notexhausted, the MS retransmits the MOB-SCN-REQ message to the BS afterresetting the timer. If the scan response (MOB-SCN-RSP) message isreceived from the BS prior to expiration of the scan response standbytimer, the MS performs a scanning operation according to informationincluded in the corresponding response message.

FIG. 11 is a signal flow diagram illustrating sleep request and responsemessages using a sleep response standby timer, according to anembodiment of the present invention.

Referring to FIG. 11, an MS enables a sleep response standby timer upontransmitting a sleep request (MOB-SLP-REQ) message to a BS. If a sleepresponse (MOB-SLP-RSP) message is not received before the sleep responsestandby timer expires, the MS checks whether the sleep requestretransmission count is exhausted. If the sleep request count is notexhausted, the MS retransmits the MOB-SLP-REQ message to the BS afterresetting the timer. If the sleep response (MOB-SCN-RSP) message isreceived from the BS prior to expiration of the sleep response standbytimer, the MS performs a sleep mode operation according to informationincluded in the corresponding response message.

FIG. 12 is a signal flow diagram illustrating de-registration requestand command messages using a de-registration command standby timeraccording to an embodiment of the present invention (e.g., a case wherethe de-registration request message transmitted from a mobile station islost).

Referring to FIG. 12, an MS enables a de-registration command standbytimer upon transmitting a de-registration request (DREG-REQ) message toa BS. If a de-registration command (DREG-CMD) message is not receivedbefore the de-registration command standby timer expires, the MS checkswhether the de-registration request retransmission count is currentlyexhausted. If the de-registration request retransmission count is notexhausted, the MS retransmits the DREG-REQ message to the BS afterresetting the timer. If the de-registration command (DREG-CMD) messageis received from the BS prior to expiration of the de-registrationcommand standby timer, the MS performs an idle mode operation accordingto information included in the corresponding response message.

FIG. 13 is a signal flow diagram illustrating de-registration requestand command messages using a de-registration command standby timeraccording to another embodiment of the present invention (e.g., a casewhere the de-registration command message transmitted from a basestation is lost).

Referring to FIG. 13, a BS enables a management resource holding timerupon transmitting a de-registration command (DREG-CMD) message to a BS.If the DREG-CMD message is lost, e.g., lost over radio, the MSretransmits the DREG-REQ message via basic connection held by themanagement resource holding timer after the de-registration commandstandby timer expires. The BS retransmits the DREG-CMD message andresets the management resource holding timer. If the management resourceholding timer expires after the DREG-CMD message has been transmittednormally, the BS decides that the MS has entered an idle mode and thenreleases management resources.

FIG. 14 and FIG. 15 are flow diagrams illustrating a method according toanother embodiment of the present invention. Referring to FIGS. 14 and15, an exemplary case is shown where, if a MS does not request idle modeinitialization via a DREG-REQ message, an unsolicited de-registrationcommand (DREG-CMD) message transmitted by a BS to a MS is lost.

The BS enables a management resource holding timer and a de-registrationcommand standby timer to verify whether a previously set timer expiresupon transmitting the unsolicited DREG-CMD message to the MS. If ade-registration response (DREG-REQ in FIG. 14 or DREG-ACK in FIG. 15)message is not received as an acknowledgement response for theunsolicited DREG-CMD message from the MS before the de-registrationresponse standby timer expires, the BS checks whether thede-registration command retransmission count is currently exhausted. Ifthe de-registration command count is not exhausted, the BS retransmitsthe unsolicited DREG-CMD message to the MS after resetting thede-registration command standby timer and resets the management resourceholding timer. Thereafter, if a de-registration response (DREG-REQ orDREG-ACK) from the MS is received before the de-registration responsestandby timer expires, the BS releases management resources that havebeen held for the MS after the management resource holding timerexpires.

FIG. 14 illustrates a case where the MS modifies the formerde-registration request message to transmit the de-registration responsemessage, as illustrated in the example in Table 6, to transmit thede-registration response message. FIG. 15 illustrates a case where thede-registration response message is transmitted via the DREG-ACKmessage, as illustrated in the example in Table 7.

In one embodiment, a method for transmitting messages to alter theoperating status between a mobile station and a base station in awireless access system comprises, at the mobile station, transmitting afirst message to the base station, setting a first count to a firstpredetermined value and enabling a first, the first message requesting aalteration in operating status between the mobile station and the basestation, whereupon expiration of the first timer before receiving asecond message from the base station, the count is decremented and, ifthe count is above a second predetermined value, the first timer isreset and the first message is retransmitted to the base station. Themethod also comprises, at a base station, receiving the first message,transmitting a second message to the mobile station and enabling asecond timer, the second message acknowledging the mobile stationrequest for an alteration in operating status, whereupon expiration ofthe second timer, the alteration in operating status is effected andupon receiving another first message, the second message isretransmitted to the mobile station and the second timer is reset.

The first message may be a de-registration request (DREG-REQ), the firsttimer is a de-registration command standby timer, the first count is ade-registration request retransmission count, the alteration inoperating status is a transition to an idle mode, the second message isa de-registration command (DREG-CMD), and the second timer is amanagement resource holding timer. The method may further comprise, atthe mobile station, performing a scan operation in response to receivingthe second message from the base station. The method may furthercomprise, at the mobile station, entering idle mode in response toreceiving the second message from the base station.

In another embodiment, a method for altering the operating statusbetween a mobile station and a base station in a wireless access systemcomprises receiving a first message, the first message requesting thealteration in operating status. The method also comprises transmitting asecond message, the second message acknowledging the request for analteration in operating status. The method also comprises enabling atimer, and upon expiration of the timer, effecting the alteration inoperating status and upon receiving another first message,retransmitting the second message and resetting the timer.

The first message may be a de-registration request, the alteration inoperating status is a transition to an idle mode, the second message isa de-registration command, and the timer is a management resourceholding timer.

In yet another embodiment, a method for altering the operating statusbetween a mobile station and a base station in a wireless access systemcomprises transmitting a first message, the first message indicating thealteration in operating status. The method also comprises enabling afirst timer and setting a count to a first predetermined value. Themethod also comprises enabling a second timer, whereupon expiration ofthe first timer before receiving a second message, the count isdecremented and, if the count is above a predetermined value, the firsttimer is reset and the first message is retransmitted and, whereuponexpiration of the second timer, the alteration in operating status iseffected.

The first message may be an unsolicited de-registration command, thealteration in operating status is a transition to an idle mode, thesecond message is one of a de-registration request and a de-registrationacknowledgment, the first timer is a de-registration response standbytimer, and the second timer is a management resource holding timer.

Accordingly, the present invention provides an effective processingprocedure if messages are lost due to limited radio link resourcesand/or environmental radio factors in a wireless access system, therebypreventing malfunction in the MS or BS. Specifically, the presentinvention transmits the scan, sleep and de-registration request messagesand operates response standby timers. Thus, even if the request messagesor the response messages are lost, the corresponding request messagesare retransmitted after expirations of the timers.

It will be apparent to those skilled in the art that variousmodifications and variations may be made in the present inventionwithout departing from the spirit or scope of the inventions. Thus, itis intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

1-16. (canceled)
 17. A method for performing a scanning procedure by amobile subscriber station, the method comprising: transmitting a scanrequest message to a base station, the scan request message comprising ascan duration, an interleaving interval, a scan iteration, and an HMACTuple; enabling a scan response standby timer; checking whether a scanresponse message is received before the scan response standby timerexpires, the scan response message comprising a scan duration, a reportmode, and a scan report period; retransmitting the scan request messageto the base station and resetting the scan response standby timer if thescan response message is not received before the scan response standbytimer expires; and performing a scanning operation according toinformation included in the scan response message if the scan responsemessage is received from the base station prior to expiration of thescan response standby timer.
 18. The method of claim 17, furthercomprising considering the scan request message or the scan responsemessage lost if the scan response message is not received before thescan response standby timer expires.
 19. The method of claim 17, whereinthe scan response standby timer is set to a time for standby untilretransmitting the scan request message if the scan response message isnot received.
 20. The method of claim 17, wherein the scan responsestandby timer is set to second.
 21. The method of claim 17, furthercomprising: setting a scan request retransmission count; checkingwhether the scan request retransmission count is exhausted; and notretransmitting the scan request message to the base station if the scanrequest retransmission count is exhausted.
 22. The method of claim 21,wherein the scan request retransmission count is an availableretransmission count of the scan request message.
 23. The method ofclaim 21, wherein the scan request retransmission count is set to 3.